The present invention pertains to a sensor for measuring O2 concentrations in liquids, with a working electrode, with a counterelectrode and with a reference electrode, wherein the working electrode and the counterelectrode are in contact with the liquid, wherein the reference electrode is separated from the liquid via a diaphragm and wherein the reference electrode measures a polarization voltage effectively acting at the working electrode and is connected to a potentiostat regulating the potential between the working electrode and the counterelectrode.
Oxygen in the molecular form, dissolved in liquids, is undesirable in many chemical and/or food technological processes. For example, very low oxygen concentrations are necessary for reasons of purity and/or corrosion protection in the manufacture of semiconductors or in the water-steam cycle in power plants. Low oxygen contents are desirable in food technology for reasons of good shelf life. For example, only very little oxygen may be present in the end product at most in the area of beer-brewing if satisfactory stability of the taste is to be achieved. The accurate measurement and monitoring of oxygen concentrations is therefore of broad analytical significance.
A sensor of the design mentioned in the introduction has been known from the reference source Brauindustrie, 3/88. This is a three-electrode sensor operating by amperometric measurement with a working electrode, a counterelectrode as well as a reference electrode. The working electrode and the counterelectrode are arranged coaxially to one another and the liquid flows through the intermediate space between the working electrode and the counterelectrode. The reference electrode is arranged on the side and is connected via a diaphragm passing through the reference electrode.
The basic functions of the sensor known so far and also of a sensor according to the present invention are as follows. An electric current between the cathodically polarized working electrode and the counterelectrode, which results from the reduction of O2 molecules at the working electrode, is measured. It is obvious that reduction of O3 molecules can also take place and the sensors known so far, just as a sensor according to the present invention, can be used to determine ozone or total (molecular) oxygen. On the one hand, accurate setting of the potential between the working electrode and the counterelectrode is necessary for the measurement of the current intensity. On the other hand, determination of the rate of flow of the liquid must be performed if the measurement is to be performed in flowing liquids, because the measured values obtained are also determined by the thickness of the boundary layer at the working electrode in which the O2 transport takes place in a diffusion-controlled manner. The thickness of the boundary layer is in turn a function of the rate of flow.
A method for determining the rate of flow is, e.g., the technique known from practice in which two NTC resistors are used. One NTC resistor is heated and is arranged in the flow of the liquid. The second NTC resistor is likewise arranged in the liquid, but in an area without flow or only low flow. The second NTC resistor is used as a compensating resistor concerning the temperature of the liquid, while the voltage drop over the first NTC resistor (in case of temperature compensation) is an indicator of the xe2x80x9ccooling effectxe2x80x9d of the flowing liquid and therefore also of the rate of flow. It is obvious that the amount of liquid is so large that no appreciable increase in the temperature of the liquid takes place as a whole due to the heating of the first NTC.
In conjunction with a potentiostat, the reference electrode ensures that the electrochemical conditions are maintained at a constant value between the working electrode and the counterelectrode. A suitable circuit can be found, e.g., in the reference source Handbuch der industriellen Mexcex2technik [Handbook of Industrial Measuring Technique], 6th edition, 1994, R. Oldenbourg Verlag, Munich, Vienna, p. 1102.
One disadvantage of a sensor according to the above-mentioned state of the art is that it has a protruding design. Due to the fact that the working electrode and the counterelectrode, on the one hand, and the reference electrode, on the other hand, are arranged side by side, there also arises a disturbing effort to contact the sensor as a whole. In addition, a compact conductor system is necessary, which leads to a relatively great pressure drop and a considerable risk for microbial contamination. In addition, sterilization is cumbersome.
The basic technical object of the present invention is to provide a sensor of the design mentioned in the introduction, which has a compact design, has a reduced risk for microbial contamination and can be sterilized in a simple manner.
To accomplish this object, the present invention teaches that the reference electrode, the working electrode and the counterelectrode are coaxial to one another, the working electrode and the counterelectrode being arranged around the reference electrode. In other words, the sensor has an essentially rotationally symmetrical design, with the reference electrode forming the middle part.
The liquid can be easily passed through between the working electrode and the counterelectrode or past the working electrode and the counterelectrode, without the need for longer lines of a narrow cross section. The system can be rather designed as an open system, e.g., as a cylindrical system, wherein the liquid can flow through at least one front surface of the cylinder which is open over its full area.
A very compact design is obtained with the present invention due to the coaxial arrangement of all electrodes. Other advantages are a small pressure drop, reduced risk for microbial contamination, simpler sterilizability, simpler manufacturability, and reduced maintenance requirement.
When viewed in the radial direction, the working electrode and the counterelectrode may follow each other, in principle, in any desired arrangement. The counterelectrode is preferably arranged around the working electrode. The working electrode or the counterelectrode may form an outer wall of the reference electrode and thus form a structural unit.
The electrodes may have, in principle, any shape, e.g., a square shape relative to their cross section at right angles to the axis. However, the counterelectrode and/or the working electrode are preferably of a cylinder jacket design, i.e., they have a circular cross section for manufacturing technical reasons.
The reference electrode is, e.g., a class 2 electrode with constant input potential, preferably an Ag/AgCl electrode. The material of the working electrode may be silver or a silver alloy. The material of the counterelectrode may be a stainless steel.
All electrical contacts for the connection of the working electrode, the counterelectrode and the reference electrode may be arranged at one end of the axis of the sensor. As a result, simple contactability and higher electrical reliability are achieved.
A coaxial opening, through which the liquid can flow, preferably a coaxial opening extending over the entire cross section of the sensor, may be arranged at the end of the axis of the sensor located opposite the contacts. In conjunction herewith, a radial opening, through which the liquid can flow, may be arranged in the electrode that is the outermost electrode in the radial direction. For example, liquid can be passed through the radial opening into the interior of the sensor, and the coaxial opening functions as an outflow opening in this case. If the electrical contacts are not arranged in the area of one end of the axis of the sensor (but, e.g., laterally or radially), a coaxial opening of the above-mentioned design each may be arranged at both ends. An extremely low flow resistance is achieved in this case.
A temperature sensor, which is in thermal contact with the liquid, preferably an NTC resistor, may be arranged between the counterelectrode and the working electrode, i.e., in the intermediate space of the sensor through which the liquid flows. This temperature sensor can be used solely to measure the temperature of the liquid, e.g., within the framework of a temperature compensation of an arrangement for measuring a rate of flow, but it may also be used itself to measure the rate of flow through the sensor (with or without external temperature compensation). It is, of course, also possible to use other devices for measuring rates of flow. The examples include pressure drop measurement over a standard diaphragm, a standard nozzle or a venturi tube, the float element method, turbine flow meters, impeller flow meters, oval wheel flow meters, ultrasonic flow meters and flow measurement by magnetic induction.
The other designs described below are of significance in themselves. The additional elements described may represent, on the one hand, an (external) calibrating means for calibrating a sensor of the above-described design. However, the overall design obtained can also be used as a sensor, in which case there is a possibility for xe2x80x9cin situxe2x80x9d calibration.
A venturi nozzle may be connected to the coaxial opening. A flow chamber with an opening through which the liquid can flow, which is preferably designed as a second radial opening, may be connected on the side of the venturi nozzle located opposite the coaxial opening. The flow chamber may include an electrolysis cell. A heatable temperature sensor, preferably an NTC resistor, may be arranged for flow measurement or for the measurement of the rate of flow in the area of the venturi nozzle. However, this may also be determined, e.g., from the pressure drop.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.